The present invention relates to tunable lasers, in general, and more particularly to a rapidly tunable laser system operative to tune the laser beam emission to a predetermined wavelength in accordance with the polarization state thereof, whereby the laser beam emission may be tuned between predetermined wavelengths as rapidly as it can be altered between polarization states.
Recent target recognition systems are measuring atmospheric pollution for the purposes of detecting and classifying certain targets. These systems generally include a laser system having emissions which are alternately tunable between at least two frequencies, preferably widely separated in wavelength. For the cases in which effluents are measured as the atmospheric pollution, one laser beam emission wavelength may be tuned to the center of the absorption line of an effluent of interest, while another laser beam emission wavelength may be tuned to a reference line whose attenuation is virtuely unaffected by the presence or absence of the effluent to be measured. In order to minimize the effects of atmospheric disturbances, like turbulence, for example, the laser beam emissions of the one and another wavelengths should be pulsedly transmitted alternately as closely spaced in time as possible. Additional minimum pulse repetition rate requirements may be imposed in military applications where fast acquisition and detection times are mandatory to avoid detection by an enemy in a hostile environment.
Tunable laser systems which have been developed over the past several years typically use mechanically movable intracavity elements such as diffraction gratings, prisms or etalons, for example, as the frequency tuning elements. Representative movable element tunable laser systems are disclosed in the U.S. Patents listed directly herebelow:
U.S. Pat. No. 3,983,507 issued to Tang et al. on Sept. 28, 1976 and entitled "Tunable Laser Systems and Method"; PA1 U.S. Pat. No. 3,983,058 issued to Yamamoto on Feb. 10, 1976 and entitled "Tunable Laser"; PA1 U.S. Pat. No. 3,857,109 issued to Pilloff on Dec. 24, 1974, and entitled "Longitudinally-Pumped Two-Wavelength Lasers"; and PA1 U.S. Pat. No. 3,790,898 issued to Gudmundsen et al. on Feb. 5, 1974, and entitled "Selectively Tunable Gaseous Laser". PA1 U.S. Pat. No. 4,122,412 issued to Hughes on Oct. 24, 1978, and entitled "Magneto-Optically Tuned Lasers"; PA1 U.S. Pat. No. 4,028,636 issued to Hughes on June 7, 1977, and entitled "Acousto-Optical Deflector Tuned Organic Dye Laser"; PA1 U.S. Pat. No. 3,991,383 issued to Hughes on Nov. 9, 1976, and entitled "Franz-Keldysh Effect Tuned Laser"; and PA1 U.S. Pat. No. 3,959,739 issued to Hughes et al on May 25, 1976, and entitled "Electro-Optic Tuning of Organic Dye Laser".
In the laser systems represented by the referenced patents hereabove, the tuning of the laser beam emission from one frequency to another requires the physical mechanical motion of one or more of the intracavity elements of the laser system. The physical motion of an element from one position to another usually cannot be brought about rapidly and thus is a primary limitation to the rate of tuning. Another disadvantage of the mechanical scanning of an intracavity element is that adjacent laser lines (i.e. specific wavelengths emitted by the laser) cannot be skipped over in the tuning process between widely separated desired wavelengths. As a result, unwanted wavelengths may be emitted from the laser system which could possibly alter or confuse the precision analytical measurements being derived. In addition, physical motion of the intracavity elements are known to frequently result in both amplitude and frequency instabilities of the laser beam emission which may further contribute to the inaccuracies of the precision measurements taking place.
Other laser systems which do not use movable intracavity elements in the tuning process generally include an optical device which is responsive to either acoustic, electric or magnetic inputs to deflect the laser beam from the optical axis of the laser cavity to an angle of incidence with a reflective device, normally a diffraction grating, the incident angle being governed by the input signal of the diffraction cell. In these laser systems, the reflecting device is designed to retroreflect laser radiation of a unique wavelength dependent on the angle of incidence thereof.
Examples of laser systems employing non-movable intracavity optical elements for tuning between specific laser beam wavelengths are disclosed in the following U.S. Patents:
In these references, the tuning operation involves physically deflecting the radiation beam to strike different portions of the tuning or reflective optical element at corresponding angles of incidence (i.e. for each wavelength desired, a different portion of the tuning optical element is used for retroreflection). Needless to say, this technique requires a uniform fabrication process in making the tuning optical element, especially over the surface area which is used by the radiation beam, to ensure a uniform optical response.
The laser system described in the specification herebelow overcomes the undesirable and disadvantageous aspects of the aforementioned laser systems and provides rapid tuning between preselected laser beam wavelengths.